Controllable transition of silk fibroin nanostructures: an insight into in vitro silk self-assembly process

Use of Bombyx mori silk fibroin in tissue engineering: From cocoons to medical devices, challenges, and future perspectives

Silk fibroin has become a prominent material in tissue engineering (TE) over the last 20 years with almost 10,000 published works spanning in all the TE applications, from skeleton to neuronal regeneration. Fibroin is an extremely versatile biopolymer that, due to its ease of processing, has enabled the development of an entire plethora of materials whose properties and architectures can be tailored to suit target applications. Although the research and development of fibroin TE materials and devices is mature, apart from sutures, only a few medical products made of fibroin are used in the clinical routines. <40 clinical trials of Bombyx mori silk-related products have been reported by the FDA and few of them resulted in a commercialized device. In this review, after explaining the structure and properties of silk fibroin, we provide an overview of both fibroin constructs existing in the literature and fibroin devices used in clinic. Through the comparison of these two categories, we identified the burning issues faced by fibroin products during their translation to the market. Two main aspects will be considered. The first is the standardization of production processes, which leads both to the standardization of the characteristics of the issued device and the correct assessment of its failure. The second is the FDA regulations, which allow new devices to be marketed through the 510(k) clearance by demonstrating their equivalence to a commercialized medical product. The history of some fibroin medical devices will be taken as a case study. Finally, we will outline a roadmap outlining what actions we believe are needed to bring fibroin products to the market.

Adsorption, Surface Viscoelasticity, and Foaming Properties of Silk Fibroin at the Air/Water Interface

Like other proteins, the natural silk fibroin (SF) extracted from domesticated silkworms can adsorb at the air/water interface and stabilize foam due to its amphiphilic character and surface activity. At the interface, the adsorbed SF molecules experience structural reorganization and form water-insoluble viscoelastic films, which protect foam bubbles from coalescence and rupture. The solution conditions, such as protein concentration, pH, and additives, have significant influences on the molecular adsorption, layer thickness, interfacial mechanical strength, and, thus, on the foaming properties of SF. The understanding of the relationship between the interfacial adsorption, surface viscoelasticity, and foaming properties of SF is very important for the design, preparation, and application of SF foams in different fields.

Fabrication of silk fibroin film enhanced by acid hydrolyzed silk fibroin nanowhiskers to improve bacterial inhibition and biocompatibility efficacy

In this study, silk fibroin nanowhiskers (SNWs) were extracted from natural silk fiber by sulfuric acid hydrolysis with the assistance of ultrasonic wave treatment. The obtained SNWs were mixed with regenerated silk fibroin (RSF) solution to fabricate the SNWs/RSF films. The fabricating SNWs were systematically characterized by using SEM, FTIR, and the SNWs/RSF films were observed by digital camera, PM, etc. The results show that the monodisperse SNWs are evenly distributed in the RSF film. The presence of SNWs in RSF film significantly improves the performances of the film, including the swelling ability, mechanical properties, hydrophilicity, antibacterial efficacy, cytocompatibility. Meanwhile, the SNWs/RSF film can endorse the wound healing efficiency in vivo mice wound site. The proposed techniques for extracting SNWs and fabricating silk fibroin composite film may provide a valuable method for creating an ideal silk-based material for biomedical applications.

A sonication-induced silk-collagen hydrogel for functional cartilage regeneration

Cartilage tissue has limited self-regeneration capacity and current treatment methods often result in fibrocartilage formation. Although collagen has shown the ability to induce chondrogenesis of mesenchymal stem cells (MSCs) and regenerate hyaline cartilage, the application of a pure collagen hydrogel is inherently limited by its fast degradation, poor mechanical properties and excessive cell-mediated shrinkage. To overcome this challenge, we developed a sonication-induced silk-collagen composite hydrogel (COL + SF(S)) and investigated its physicochemical and biological properties compared with a collagen hydrogel (COL) and a non-sonicated silk-collagen composite hydrogel (COL + SF(NS)). The results showed that the sonication treatment of silk fibroin induced antiparallel β-sheet formation and a stronger negative charge on the silk fibroin molecule, which resulted in improved mechanical properties of the COL + SF(S) hydrogel. The COL + SF(S) hydrogel exhibited superior stability during cell culture and promoted the gene expression of SOX9 at the early stage and sulfated glycosaminoglycan (sGAG) deposition without any exogenous growth factor. Moreover, the cartilage regeneration capacity of the COL + SF(S) group was evaluated in rabbit knee defects. The COL + SF(S) group exhibited well-integrated articular hyaline cartilage closely resembling native articular cartilage after 6 months. Overall, the COL + SF(S) hydrogel holds great potential as a scaffold material to regenerate functional hyaline cartilage.

Electrospun SF/PLGA/ICG Composite Nanofibrous Membranes for Potential Wound Healing and Tumor Therapy

Indocyanine green (ICG) is a near-infrared (NIR) organic reagent for clinical bioimaging and phototherapy. It is a suitable photosensitizer for photodynamic antimicrobial chemotherapy (PACT). In this study, various ICG-loaded nanofibrous membranes were prepared. The water vapor transmission rate (WVTR) of SF/PLGA/20ICG was 3040.49 ± 157.11 g·m−2 day−1, which allowed the maintenance of a humid environment above the wound. The growth inhibition rates for S. aureus and E. coli were 91.53% and 87.95%, respectively. The nanofibrous membranes exhibited excellent antimicrobial performance. Cellular experiments showed that the nanofibrous membranes have good cytocompatibility and antitumor efficacy. SF/PLGA/20ICG showed good potential for application in wound healing and cancer therapy.

Mimicking Native Heart Tissue Physiology and Pathology in Silk Fibroin Constructs through a Perfusion-Based Dynamic Mechanical Stimulation Microdevice

In vitro cardiomyocyte (CM) maturation is an imperative step to replicate native heart tissue-like structures as cardiac tissue grafts or as drug screening platforms. CMs are known to interpret biophysical cues such as stiffness, topography, external mechanical stimulation or dynamic perfusion load through mechanotransduction and change their behavior, organization, and maturation. In this regard, a silk-based cardiac tissue (CT) coupled with a dynamic perfusion-based mechanical stimulation platform (DMM) for achieving maturation and functionality in vitro is tried to be delivered. Silk fibroin (SF) is used to fabricate lamellar scaffolds to provide native tissue-like anisotropic architecture and is found to be nonimmunogenic and biocompatible allowing cardiomyocyte attachment and growth in vitro. Further, the scaffolds display excellent mechanical properties by their ability to undergo cyclic compressions without any deformation when places in the DMM. Gradient compression strains (5% to 20%), mimicking the native physiological and pathological conditions, are applied to the cardiomyocyte culture seeded on lamellar silk scaffolds in the DMM. A strain-dependent difference in cardiomyocyte maturation, gene expression, sarcomere elongation, and extracellular matrix formation is observed. These silk-based CTs matured in the DMM can open up several avenues toward the development of host-specific grafts and in vitro models for drug screening.

Tunable microphase-regulated silk fibroin/poly (lactic acid) biocomposite materials generated from ionic liquids

One of the most effective and promising strategies to develop novel biomaterials with unique, tunable structure and physicochemical properties is by creating composite materials that combine synthetic polymers with natural proteins using ionic liquids. In this study, biodegradable poly(d,l-lactic acid) (PDLLA) was blended with silk fibroin (SF) to create biocompatible films using an ionic liquid-based binary solvent system (1-butyl-3-methylimidazolium chloride/N,N-dimethylformamide), which can maintain the molecular weights of the proteins/polymers and encourage intermolecular interactions between the molecules. The effects of varying the ratio of PLA to SF were studied using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), water contact angle testing, and cytotoxicity analysis as well as enzymatic degradation. Results showed that the composite films were homogeneously blended on the macroscopic scale and exhibited typical fully miscible polymer blend characteristics. By increasing the SF content in the composites, the amounts of β-sheets in the films were significantly increased, allowing for SF to act as a physical crosslinker to maintain the stability of the protein-polymer network. Additionally, SF significantly improved the hydrophilicity and biocompatibility of the material and promoted the self-assembly of micelle structures in the biocomposites. Different topologies in the films also provided beneficial surface morphology for cell adhesion, growth, and proliferation. Overall, this study demonstrated an effective fabrication method for a fine-tuned polymer blends combining synthetic polymer and protein for a wide variety of biomedical and green material applications.

The preparation and study on properties of calcium sulfate bone cement combined tuning silk fibroin nanofibers and vancomycin-loaded silk fibroin microspheres

In this study, a bioactive composite material based on calcium sulfate hemihydrate (CSH) bone cement was studied, which use calcium sulfate dihydrate (CSD) as coagulant and silk fibroin nanofibers (SFF) solution as the curing liquid, further loaded vancomycin silk fibroin microspheres (SFM/VCM). The drug release effect of bone cements caused by tuning weight content of SFM/VCM (0.5, 1, 2%) and the concentration of silk fibroin solution (SFS) (20, 60, 100 mg/mL) used for preparation of SFM was studied in this article. Scanning electron microscope (SEM) demonstrated that the average diameter of microspheres gradually increased and the setting time was prolonged with the concentration of SFS increasing. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) were used to analyze the composition of composite materials. The result of compressive strength revealed that the composites contained 0.5% SFM/VCM showed better mechanical performance independent on the concentration of microspheres and the cumulative drug release percentage of all composites were less than 55% after 4 weeks. The drug-loading bone cement possesses not only injectability but also sustained release capability which has a promising prospect in the field of bone substitute material.

Boost of the Bio-memristor Performance for Artificial Electronic Synapses by Surface Reconstruction

Biomaterial-based memristors (bio-memristors) are often adopted to emulate biological synapse functions and applied to construct neural computing networks in brain-inspired chip systems. However, the randomness of conductive filament formation in bio-memristors inhibits their switching performance by causing the dispersion of the device-switching parameters. In this case, a facile porous silk fibroin (p-SF) memristor was obtained through a protein surface reconstruction strategy, in which the size of the hole can be adjusted by the density of hybrid nanoseeds. The porous SF memristors exhibit greatly enhanced electrical characteristics, including uniform I-V cycles, centralized distribution of the switching voltages, and both high and low resistances, compared to devices without pores. The results of three-dimensional (3D) simulations based on classical density functional theory (cDFT) suggest that the reconstructed pores in the SF layers guide the formation and fracture of Ag filaments under an electric field and enhance the overall conductivity by separating Ag⁺ ion and electron diffusion pathways. Ag⁺ ions are predicted to preferentially diffuse through pores, whereas electrons diffuse through the SF network. Interestingly, the device conductance can be bidirectionally modulated gradually by positive and negative voltages, can faithfully simulate short-term and long-term plasticity, and can even realize the triplet-spike-timing-dependent plasticity (triplet-STDP) rule, which can be used for pattern recognition in biological systems. The simulation results reveal that a memristor network of this type has an accuracy of ∼95.78% in memory learning and the capability of pattern learning. This work provides a facile technology route to improve the performance of bionic-material memristors.

Electrospun PLGA/SF/artemisinin composite nanofibrous membranes for wound dressing

Combining biodegradable materials with natural plant components for wound dressing has been receiving significant attention. ART is a sesquiterpene lactone compound extracted from Artemisia annua L., possessing multiple pharmacological effects including antibacterial activity and anti-inflammatory property. Herein, the blended polylactic acid glycolic acid (PLGA)/silk fibroin (SF) membranes loaded with artemisinin (ART) are fabricated through electrospinning. With aid of SF, the fabricated membranes have a good sustained-release effect, and the accumulated ART release can reach 69% after three weeks. PLGA/SF/ART membranes exhibit favorable anti-inflammatory and cell compatibility in vitro evaluations. The in vivo experiment indicates that PLGA/SF/ART2 membranes can shorten the inflammation period and enhance skin regeneration in a full-thickness wound model through down-regulating the expressions of pro-inflammatory cytokines IL-1β and TNF-α. To sum up, the fabricated PLGA/SF/ART2 composite membranes with anti-inflammatory properties can be a proposal wound dressing for chronic wound healing.

Processing, mechanical properties and bio-applications of silk fibroin-based high-strength hydrogels

Hydrogels are an attractive class of materials that possess similar structural and functional characteristics to wet biological tissues and demonstrate a diversity of applications in biomedical engineering. Silk fibroin (SF) is a unique natural polymer due to its fibrous protein nature, versatile formats, biocompatibility, tunable biodegradation and is thus a good hydrogel candidate for bio-applications. Compared to synthetic polymer hydrogels, poor mechanical performance is still a fatal drawback that hinders the application of SF hydrogels as structural materials. Researchers have attempted to develop strategies to construct silk fibroin-based high-strength hydrogels (SF-HSHs). Herein, we firstly provide an overview of the approaches of processing SF-HSHs with a focus on the physical/non-covalent crosslinking mechanisms. The examples of SF-HSHs are discussed in detail under four categories, including physical-crosslinked, dual-crosslinked, double network and composite hydrogels respectively. A brief section follows to elucidate on the gelation mechanisms of SF-HSHs before a description of the utility of SF-HSHs in biomedicine and devices is presented. Finally, the potential challenges and future development of SF-HSHs are briefly discussed. This review aims to enhance our understanding of the structure-mechanical property-function relationships of soft materials made from natural polymers and guide future research of silk fibroin-based hydrogels for biomedical applications. STATEMENT OF SIGNIFICANCE: Silk fibroin (SF) extracted from silk fibres is increasingly applied in the biomedical field, and SF hydrogel has been an emerging area for frontier bio-research. Since SF biopolymer has an intrinsic tendency to form regular β-sheet stacks, it can be processed into purely physically crosslinked hydrogels, thus avoiding the use of chemical crosslinkers. Nevertheless, akin to other natural polymers, lab-produced SF is variable (i.e. the molecular weight and distribution), and the gelation of SF hydrogel is challenging to control. In addition, hydrogels made from SF are usually weak and brittle, which hinders the wide use of this biofriendly and biodegradable hydrogel. Recently, there is a pressing need for high strength hydrogels from natural polymers for biomedical applications, and SF is proposed as a strong candidate. Therefore, we have studied the literature in the past 10 years and would like to focus on the gelation mechanism and mechanical strength of SF hydrogels for the review.

Dissolution and processing of silk fibroin for materials science

In recent decades, silk fibroin (SF) from silkworm Bombyx mori has been extensively researched and applied in several fields, including: cosmetics, biomedicine and biomaterials. The dissolution and regeneration of SF fibers is the key and prerequisite step for the application of silk protein-based materials. Various solvents and dissolving systems have been reported to dissolve SF fibers. However, the dissolution process directly affects the characteristics of SF and particularly impacts the mechanical properties of the resulting silk biomaterials in subsequent processing. The purpose of this review is to summarize the common solvents, the dissolution methods for silk protein, the properties of the resulting SF protein. The suitable use of SF dissolved in the corresponding solvent was also briefly introduced. Recent applications of SF in various biomaterials are also discussed.

A mussel-inspired supramolecular hydrogel with robust tissue anchor for rapid hemostasis of arterial and visceral bleedings

In recent years, the developed hemostatic technologies are still difficult to be applied to the hemostasis of massive arterial and visceral hemorrhage, owing to their weak hemostatic function, inferior wet tissue adhesion, and low mechanical properties. Herein, a mussel-inspired supramolecular interaction-cross-linked hydrogel with robust mechanical property (308.47 ± 29.20 kPa) and excellent hemostatic efficiency (96.5% ± 2.1%) was constructed as a hemostatic sealant. Typically, we combined chitosan (CS) with silk fibroin (SF) by cross-linking them through tannic acid (TA) to maintain the structural stability of the hydrogel, especially for wet tissue adhesion ability (shear adhesive strength = 29.66 ± 0.36 kPa). Compared with other materials reported previously, the obtained CS/TA/SF hydrogel yielded a lower amount of blood loss and shorter time to hemostasis in various arterial and visceral bleeding models, which could be ascribed to the synergistic effect of wound closure under wet state as well as intrinsic hemostatic activity of CS. As a superior hemostatic sealant, the unique hydrogel proposed in this work can be exploited to offer significant advantages in the acute wound and massive hemorrhage with the restrictive access of therapeutic moieties.

Top-down extraction of surface carboxylated-silk nanocrystals and application in hydrogel preparation

Bionanomaterial based hydrogels originated from natural biopolymer have drawn much attention for advanced applications. However, nanosilk-based hydrogels derived from top-down approaches remain in their infancy. First, nanosilks based on existing methods fail to prepare hydrogels; second, both nanosilk extraction and surface modification remain a challenge due to high crystallinity and sophisticated hierarchical structures. To produce nanosilk-based hydrogels, pretreatment and oxidation are necessary. In this work, pretreatments were conducted first to loosen the sophisticated structures of natural silk fibers, NaClO oxidation was utilized in succession to introduce carboxyl groups onto silk fibroin. Combined with moderate mechanical disintegration, silk nanocrystals with additional carboxyl groups were prepared facilely. Finally, silk nanocrystal-based hydrogels were prepared successfully through gas phase coagulation. An optimization of pretreatment approaches and oxidation conditions was carried out. The morphologies, chemical and crystalline structures of original, pretreated and oxidized silk fibroin as well as nanofibrillated silk were investigated. In addition, the silk nanocrystal-based hydrogel exhibited outstanding mechanical properties compared to those of dissolved and regenerated silk fibroin-based hydrogels. Moreover, silk nanocrystal-based aerogels present highly porous, interconnected, and crisscrossed network nanostructures, which are ideal candidates for tissue regeneration and provide new prospects as porous scaffolds for bioengineering applications.

Self-Assembly of Bombyx mori Silk Fibroin

Silk fibroin from Bombyx mori (silkworm) distinguishes for its unique mechanical performance, controllable degradation rates, and easily large-scale production, making it attractive models for a variety of biomaterial design. These outstanding properties of silk fibroin originate from its unique modular composition of silk proteins. To exploit the structure-function relationship and fabricate silk fibroin-based biomaterials, comprehensive strategies to uncover assembly behaviors of fibrous proteins are necessary. This chapter describes methods to produce regenerated silk fibroin protein from Bombyx mori silk and their self-assembly strategies. This could provide insight into the fabrication of various silk fibroin-based biomaterials, such as hydrogels, tubes, sponges, fibers, microspheres, and diverse thin film patterns, which can be used for textiles, electronics and optics, environmental engineering, and biomedical applications.

Formation, Structure, and Mechanical Performance of Silk Nanofibrils Produced by Heat-Induced Self-Assembly

The heat-induced self-assembly of silk fibroin (SF) is studied by combing fluorescence assessment, infrared nanospectroscopy, wide-angle X-ray scattering, and Derjaguin-Muller-Toporov coupled with atomic force microscopy. Several fundamental issues regarding the formation, structure, and mechanical performance of silk nanofibrils (SNFs) under heat-induced self-assembly are discussed. Accordingly, SF in aqueous solution is rod-like in shape and not micellar. The formation of SNFs occurs through nucleation-dependent aggregation, but the assembly period is variable and irregular. SF shows inherent fractal growth, and this trend is critical for the short-term assembly. The long-term assembly of SF, however, mainly involves an elongation growth process. SNFs produced by different methods, such as ethanol treatment and heat incubation, have similar secondary structure and mechanical properties. These investigations improve the in-depth understanding of fundamental issues related to self-assembly of SNFs, and thus provide inspiration and guidance in designing of silk nanomaterials.

Multi-Responsive Silk Fibroin-Based Nanoparticles for Drug Delivery

Silk fibroin has the merits of biocompatibility, biodegradability, ease of processing, and feasibility of modification, which present it as a promising drug delivery material. This review focuses on the structures of silk fibroin, the controlled transformation of secondary structures, and the formation mechanism of silk fibroin-based nanoparticles (SFNPs). We also discuss the intrinsic multi-responsive, surface functionalization, and transgenic modification of SFNPs for drug delivery.

High-Strength and High-Toughness Silk Fibroin Hydrogels: A Strategy Using Dynamic Host-Guest Interactions

Natural polymer-based hydrogels attract great attention because of their inherent biocompatibility and controllable biodegradability. However, the broad applications of these hydrogels require a combination of high mechanical strength, high toughness, fatigue resistance, as well as self-healing. The integration of this combination into one natural polymer-based hydrogel remains challenging. Here, a molecular design strategy was proposed to fabricate mechanically robust silk fibroin-based hydrogels using host-guest interactions. Silk fibroin molecules was chemically modified with cholesterol (Chol, guest) or β-cyclodextrin (β-CD, host), and host-guest interaction between Chol and β-CD moieties drove the supramolecular assemblies of hydrogels. The dissociation/reassociation behavior of host-guest complexation, serving as sacrificial bonds, endowed hydrogels with effective energy dissipation and rapid self-healing ability. The prepared silk fibroin-based hydrogels exhibited high mechanical strength, high toughness, and remarkable fatigue resistance, superior to conventional silk fibroin hydrogels. Moreover, due to reversible host-guest interactions, hydrogels achieved facile functional recovery after damage without any external stimuli. This design strategy provides an avenue to develop natural polymer-based materials with robust mechanical properties, thus broadening current hydrogel applications.

Phase Diagram and Estimation of Flory-Huggins Parameter of Interaction of Silk Fibroin/Sodium Alginate Blends

Silk fibroin (SF) and sodium alginate (SA) are natural polymers used to produce biomaterials. One of the strategies to improve the properties of these products is to prepare blends with them, which are partially miscible. Phase separation is observed, therefore, the thermodynamic analysis of this system is important to predict the final state and composition of this blends. This study explored blends with a different initial composition of SF, SA, and water (WA) at 25°C and neutral pH. After phase separation, two phases were identified, one rich in SF and other rich in SA. The Flory-Huggins parameters of interaction of polymer-solvent and polymer-polymer were estimated using the extended equation and data of phase equilibrium, their values indicates the partial miscibility of the polymers.

Inhibition of growth and lung metastasis of breast cancer by tumor-homing triple-bioresponsive nanotherapeutics

Lung metastasis of breast cancer is a leading cause of cancer-related death in women. Herein, we attempted to simultaneously inhibit the growth and lung metastasis of breast cancer by delivering quercetin (QU) using LyP-1-functionalized regenerated silk fibroin-based nanoparticles (NPs). The generated LyP-1-QU-NPs had a desirable diameter (203.2 nm) and a negatively charged surface (-12.7 mV). Interestingly, these NPs exhibited intrinsic responsibilities when triggered by various stimulating factors in the tumor microenvironment (acidic pH, reactive oxygen species, and glutathione). In vitro experiments revealed that the introduction of LyP-1 to the NP surface could significantly increase their cellular uptake efficiencies by 4 T1 cells, and facilitate their accumulation in mitochondria. Moreover, LyP-1-QU-NPs showed the strongest mitochondrial damage effect among all the treatment groups. We also found that LyP-1-QU-NPs not only exhibited excellent pro-apoptotic activities but also presented strong inhibitory effects on cell mobility (migration and invasion) through anti-glycolysis and pro-autophagy. Mice experiments confirmed that LyP-1-QU-NPs could efficiently inhibit the in situ growth of breast tumors and further restrict their lung metastasis. Collectively, our results demonstrate that LyP-1-QU-NPs, which integrates the functions of tumor cell targeting, mitochondria targeting, bioresponsive drug release, pro-apoptosis, and anti-mobility, can be developed as a promising nanotherapeutic for the effective treatment of breast cancer and its lung metastasis.

Quaternized Silk Nanofibrils for Electricity Generation from Moisture and Ion Rectification

Protein nanostructures in living organisms have attracted intense interests in biology and material science owing to their intriguing abilities to harness ion transportation for matter/signal transduction and bioelectricity generation. Silk nanofibrils, serving as the fundamental building blocks for silk, not only have the advantages of natural abundance, low cost, biocompatibility, sustainability, and degradability but also play a key role in mechanical toughness and biological functions of silk fibers. Herein, cationic silk nanofibrils (SilkNFs), with an ultrathin thickness of ∼4 nm and a high aspect ratio up to 500, were successfully exfoliated from natural cocoon fibers via quaternization followed by mechanical homogenization. Being positively charged in a wide pH range of 2-12, these cationic SilkNFs could combine with different types of negatively charged biological nanofibrils to produce asymmetric ionic membranes and aerogels that have the ability to tune ion translocation. The asymmetric ionic aerogels could create an electric potential as high as 120 mV in humid ambient air, whereas asymmetric ionic membranes could be used in ionic rectification with a rectification ratio of 5.2. Therefore, this green exfoliation of cationic SilkNFs may provide a biological platform of nanomaterials for applications as diverse as ion electronics, renewable energy, and sustainable nanotechnology.

Influence of pH on the surface and foaming properties of aqueous silk fibroin solutions

Silk fibroin (SF) adsorbs at the air/water interface, reduces the surface tension, and forms interfacial layers suppressing bubble coalescence and stabilizing foam. Variation of pH alters the inter-molecular interactions of SF in the interfacial layers and thus interfacial network formation, dilatational visco-elasticity and foaming properties. At pH 4, around the isoelectric point, the reduced electrostatic repulsion between the SF molecules results in thicker adsorbed layers, but adsorption rate, foaming rate and foam stability are lower than at pH 3 and pH 7. At the highest pH investigated (pH 7), the small aggregate size and high protein flexibility lead to the formation of more ordered and stable viscoelastic interfacial networks, which are resistant to deformation breakage and generate homogeneous, denser and more stable foams.

Visible sensing of conformational transition in model silk peptides based on a gold nanoparticles indicator

To understand protein structural transition and β-sheet formation is of importance in disparate areas such as silk protein processing and disease related β-amyloid behavior. Herein, GAGSGAGAGSGAGY (GY-14), a tetradecapeptide based on the crystallizable sequence of silk fibroin, was employed as a model peptide of the crystalline regions of silk fibroin. Due to the incorporation of tyrosine (Y), GY-14 was able to reduce Au³⁺ to Au NPs and further stabilize them without any external reducing or capping reagents to produce GY-14 stabilized Au NPs (GY-14@Au NPs). The in situ prepared GY-14@Au NPs were utilized as a built-in colorimetric indicator. The influences of specified physiological factors including decreasing the pH, the addition of calcium ions and isopropanol treatment on the self-assembly behavior of GY-14@Au NPs in aqueous solution have been studied. On the basis of transmission electron microscopy (TEM), dynamic light scattering (DLS), atomic force microscopy (AFM), Fourier transform infrared (FT-IR) spectroscopy and circular dichroism (CD) measurements, the color changes and the UV-Vis absorption peak shift of GY-14@Au NPs were attributed to the conformational change of the GY-14 peptide. The colorimetric readout can be seen with the naked eye, providing an efficient indicator to study the conformational changes of peptides exposed to various environmental stimuli.

SERS Substrate with Silk Nanoribbons as Interlayer Template

The formation of hot spots is an effective approach to improve the performance of surface-enhanced Raman scattering (SERS). Silk nanoribbons (SNRs), with a height of about 1-2 nm, and Au nanoparticles (AuNPs) were assembled by electrostatic interactions to introduce sandwich hot spot structures. These sandwich structures were optimized by tuning the ratio of SNRs and AuNPs, resulting in strong SERS signals with a sensitivity of 10^-13 M and enhancement factor (EF) of 5.8 × 10⁶. Improved SERS spectrum uniformity with relative standard deviation (RSD) about 11.2% was also achieved due to the homogeneous distribution of these hot spot structures. The inherent biocompatibility of SNRs and facile fabrication processes utilized endowed the SERS substrates significant benefits toward biomedical applications, confirmed by cytocompatibility and improved SERS bioimaging capacity in vitro. The results of this study suggest the feasibility of forming high performance bioimaging systems through the use of naturally derived materials with special nanostructures.

Formic Acid Regenerated Mori, Tussah, Eri, Thai, and Muga Silk Materials: Mechanism of Self-Assembly

Flexible and water-insoluble regenerated silk materials have caught considerable interest due to their mechanical properties and numerous potential applications in medical fields. In this study, regenerated Mori (China), Thai, Eri, Muga, and Tussah silk films were prepared by a formic acid-calcium chloride (FA) method, and their structures, morphologies, and other physical properties were comparatively studied through Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray scattering (WAXS), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermogravimetric analysis (TGA). FTIR results demonstrated that the secondary structures of those five types of silk films are different from those of their respective natural silk fibers, whose structures are dominated by stacked rigid intermolecular β-sheet crystals. Instead, intramolecular β-sheet structures were found to dominate these silk films made by FA method, as confirmed by WAXS. We propose that silk I-like structures with intramolecular β-sheets lead to water insolubility and mechanical flexibility. This comparative study offers a new pathway to understanding the tunable properties of silk-based biomaterials.

Preparation of natural amphoteric silk nanofibers by acid hydrolysis

Direct extraction of silk nanofibers (SNs) from natural silk fibers was developed via a low-intensity ultrasonic-assisted sulfuric acid hydrolysis process.

Design, Fabrication, and Function of Silk-Based Nanomaterials

Animal silks are built from pure protein components and their mechanical performance, such as strength and toughness, often exceed most engineered materials. The secret to this success is their unique nanoarchitectures that are formed through the hierarchical self-assembly of silk proteins. This natural material fabrication process in sharp contrast to the production of artificial silk materials, which usually are directly constructed as bulk structures from silk fibroin (SF) molecular. In recent years, with the aim of understanding and building better silk materials, a variety of fabrication strategies have been designed to control nanostructures of silks or to create functional materials from silk nanoscale building blocks. These emerging fabrication strategies offer an opportunity to tailor the structure of SF at the nanoscale and provide a promising route to produce structurally and functionally optimized silk nanomaterials. Here, we review the critical roles of silk nanoarchitectures on property and function of natural silk fibers, outline the strategies of utilization of these silk nanobuilding blocks, and we provide a critical summary of state of the art in the field to create silk nanoarchitectures and to generate silk-based nanocomponents. Further, such insights suggest templates to consider for other materials systems.

Fibrous Protein Self-Assembly in Biomimetic Materials

Protein self-assembly processes, by which polypeptides interact and independently form multimeric structures, lead to a wide array of different endpoints. Structures formed range from highly ordered molecular crystals to amorphous aggregates. Order arises in the system from a balance between many low-energy processes occurring due to a set of interactions between residues in a chain, between residues in different chains, and between solute and solvent. In Nature, self-assembling protein systems have evolved over millions of years to organize into supramolecular structures, optimized for specific functions, with this propensity determined by the sequence of their constituent amino acids, of which only 20 are encoded in DNA. The structural materials that arise from biological self-assembly can display remarkable mechanical properties, often as a result of hierarchical structure on the nano- and microscales, and much research has been devoted to mimicking and exploiting these properties for a variety of end uses. This work presents a review of a range of studies in which biological functions are effectively reproduced through the design of self-assembling fibrous protein systems.

Biopolymer nanofibrils: structure, modeling, preparation, and applications

Biopolymer nanofibrils exhibit exceptional mechanical properties with a unique combination of strength and toughness, while also presenting biological functions that interact with the surrounding environment. These features of biopolymer nanofibrils profit from their hierarchical structures that spun angstrom to hundreds of nanometer scales. To maintain these unique structural features and to directly utilize these natural supramolecular assemblies, a variety of new methods have been developed to produce biopolymer nanofibrils. In particular, cellulose nanofibrils (CNFs), chitin nanofibrils (ChNFs), silk nanofibrils (SNFs) and collagen nanofibrils (CoNFs), as the four most abundant biopolymer nanofibrils on earth, have been the focus of research in recent years due to their renewable features, wide availability, low-cost, biocompatibility, and biodegradability. A series of top-down and bottom-up strategies have been accessed to exfoliate and regenerate these nanofibrils for versatile advanced applications. In this review, we first summarize the structures of biopolymer nanofibrils in nature and outline their related computational models with the aim of disclosing fundamental structure-property relationships in biological materials. Then, we discuss the underlying methods used for the preparation of CNFs, ChNFs, SNF and CoNFs, and discuss emerging applications for these biopolymer nanofibrils.

Changes in Silk Feedstock Rheology during Cocoon Construction: The Role of Calcium and Potassium Ions

Variation in silk feedstocks is a barrier both to our understanding of natural spinning and biomimetic endeavors. To address this, compositional changes are investigated in feedstock specimens from the domesticated silkworm (Bombyx mori). It is found that the feedstock viscosity decreased systematically by over two orders of magnitude during cocoon construction. Potential factors such as protein concentration, molecular weight, pH, or the presence of trehalose are excluded, whereas a clear correlation appear between viscosity and the relative concentrations of Ca²⁺ and K⁺ ions. It is expected that Ca²⁺ ions would favor "salt bridges" between acidic (Asp and Glu) amino acids, leading to an increased viscosity, whereas K⁺ ions would compete for these sites, thereby reducing viscosity. Thus, these findings suggest a simple, systematic yet sophisticated control of feedstock viscosity in the silkworm, which in turn can be applied to future industrial silk production.

Growth factor-free salt-leached silk scaffolds for differentiating endothelial cells

Recently, controllable kinetic assembly was introduced into the salt-leaching process with silk proteins to form scaffolds, which achieved improvement in tuning the micro-structural and mechanical properties. Here, more control of the kinetic assembly of silk in the process was integrated into salt-leaching process, resulting in significant mechanical modification of the scaffolds generated. Both glycerol additions and treatment to concentrate the protein were used to tune hydrophilic interactions during aqueous solution processing and to reduce beta-sheet formation during the salt-leaching process. These new scaffolds showed gradient changes in elastic modulus in the range of 0.9 to 7.9 kPa. Bone marrow mesenchymal stem cells grew well and showed endothelial differentiation behavior on the scaffolds with optimized stiffness. These results indicated that the introduction of silk kinetic assembly provides an additional option for the control of porous silk scaffold properties.

Mass Production of Biocompatible Graphene Using Silk Nanofibers

Mass production of high-quality graphene dispersions under mild conditions impacts the utility of the material for biomedical applications. Various proteins have been used to prepare graphene dispersions, rare sources, and expensive prices for these proteins restrict their large-scale utility for the production of graphene. Here, inexpensive silk proteins as an abundant resource in nature were used for graphene exfoliation. The silk proteins were assembled into hydrophobic nanofibers with negative charge, and then optimized for the production of graphene. Significantly higher concentrations (>8 mg mL^-1) and yields (>30%) of graphene dispersions under ambient aqueous conditions were achieved compared with previous protein-assisted exfoliation systems. The exfoliated graphene exhibited excellent stability in water and fetal bovine serum solution, cytocompatibility, and conductivity, suggesting a promising future in biomedical and bioengineering applications.

De Novo Design of Recombinant Spider Silk Proteins for Material Applications

Spider silks are well known for their superior mechanical properties that are stronger and tougher than steel despite being assembled at close to ambient conditions and using water as the solvent. However, it is a significant challenge to utilize spider silks for practical applications due to their limited sources. Fortunately, genetic engineering techniques offer a promising approach to produce useable amounts of spider silk variants. Starting from these recombinant spider silk proteins, a series of experiments and simulations strategies are developed to improve the recombinant spider silk proteins (RSSP) material design and fabrication with the aim of biomimicking the structure-property-function relationships of spider silks. Accordingly, in this review, the authors first introduce the structure-property-function relationship of spider silks. Then, the recent progress in the genetic synthesis of RSSPs is discussed and their related multiscale self-assembly behaviors is summarized. Finally, the authors outline works utilizing multiscale modeling to assist RSSP material design.

Bioactive Silk Hydrogels with Tunable Mechanical Properties

Developing bioactive hydrogels with potential to guide the differentiation behavior of stem cells has become increasingly important in the biomaterials field. Here, silk hydrogels with tunable mechanical properties were developed by introducing inert silk fibroin nanofibers (SNF) within an enzyme crosslinked system of regenerated silk fibroin (RSF). After the crosslinking reaction of RSF, the inert SNF was embedded into the RSF hydrogel matrix, resulting in improved mechanical properties. Tunable stiffness in the range of 9-60 KPa was achieved by adjusting the amount of the added NSF, significantly higher than SNF-free hydrogels formed under same conditions (about 1 KPa). In addition, the proliferation of rat bone marrow derived mesenchymal stem cells cultured on the composite hydrogels and differentiated into endothelial cells, myoblast and osteoblast cells was improved, putatively due to the control of stiffness of the hydrogels. Bioactive and tunable silk-based hydrogels were prepared via a composite SNF and crosslinked RSF system, providing a new strategy to design silk biomaterials with tunable mechanical and biological performance.

Rapidly responsive silk fibroin hydrogels as an artificial matrix for the programmed tumor cells death

Timely and spatially-regulated injectable hydrogels, able to suppress growing tumors in response to conformational transitions of proteins, are of great interest in cancer research and treatment. Herein, we report rapidly responsive silk fibroin (SF) hydrogels formed by a horseradish peroxidase (HRP) crosslinking reaction at physiological conditions, and demonstrate their use as an artificial biomimetic three-dimensional (3D) matrix. The proposed SF hydrogels presented a viscoelastic nature of injectable hydrogels and spontaneous conformational changes from random coil to β-sheet conformation under physiological conditions. A human neuronal glioblastoma (U251) cell line was used for screening cell encapsulation and in vitro evaluation within the SF hydrogels. The transparent random coil SF hydrogels promoted cell viability and proliferation up to 10 days of culturing, while the crystalline SF hydrogels converted into β-sheet structure induced the formation of TUNEL-positive apoptotic cells. Therefore, this work provides a powerful tool for the investigation of the microenvironment on the programed tumor cells death, by using rapidly responsive SF hydrogels as 3D in vitro tumor models.

Silk nanofibril self-assembly versus electrospinning

Natural silk fibers represent one of the most advanced blueprints for (bio)polymer scientists, displaying highly optimized mechanical properties due to their hierarchical structures. Biotechnological production of silk proteins and implementation of advanced processing methods enabled harnessing the potential of these biopolymer not just based on the mechanical properties. In addition to fibers, diverse morphologies can be produced, such as nonwoven meshes, films, hydrogels, foams, capsules and particles. Among them, nanoscale fibrils and fibers are particularly interesting concerning medical and technical applications due to their biocompatibility, environmental and mechanical robustness as well as high surface-to-volume ratio. Therefore, we introduce here self-assembly of silk proteins into hierarchically organized structures such as supramolecular nanofibrils and fabricated materials based thereon. As an alternative to self-assembly, we also present electrospinning a technique to produce nanofibers and nanofibrous mats. Accordingly, we introduce a broad range of silk-based dopes, used in self-assembly and electrospinning: natural silk proteins originating from natural spinning glands, natural silk protein solutions reconstituted from fibers, engineered recombinant silk proteins designed from natural blueprints, genetic fusions of recombinant silk proteins with other structural or functional peptides and moieties, as well as hybrids of recombinant silk proteins chemically conjugated with nonproteinaceous biotic or abiotic molecules. We highlight the advantages but also point out drawbacks of each particular production route. The scope includes studies of the natural self-assembly mechanism during natural silk spinning, production of silk fibrils as new nanostructured non-native scaffolds allowing dynamic morphological switches, as well as studying potential applications. This article is categorized under:  Biology-Inspired Nanomaterials > Peptide-Based Structures  Nanotechnology Approaches to Biology > Nanoscale Systems in Biology  Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Silk nanoparticles: from inert supports to bioactive natural carriers for drug delivery

Silk proteins have been studied and employed for the production of drug delivery (nano)systems. They show excellent biocompatibility, controllable biodegradability and non-immunogenicity and, if needed, their properties can be modulated by blending with other polymers. Silk fibroin (SF), which forms the inner core of silk, is a (bio)material officially recognized by the Food and Drug Administration for human applications. Conversely, the potential of silk sericin (SS), which forms the external shell of silk, could still be considered under evaluation. At the best of our knowledge, nanoparticles based on silk sericin "alone" cannot be produced, due to its physicochemical instability influenced by extreme pH, high water solubility and temperature; for these reasons, it almost always needs to be combined with other polymers for the development of drug delivery systems. In this review, we focused on silk proteins as bioactive natural carriers, since they show not only optimal features as inert excipients, but also remarkable intrinsic biological activities. SF has anti-inflammatory properties, while SS presents antioxidant, anti-tyrosine, anti-aging, anti-elastase and anti-bacterial features. Here, we give an overview on SF or SS silk-based nanosystems, with particular attention on the production techniques.

Self-assembling oxidized silk fibroin nanofibrils with controllable fractal dimensions

The dynamic and fractal structural of silk nanofibril assembly was regulated by their surface charge distribution (carboxyl groups) and concentration.